Project Summary/abstract We have identified that increased expression of Ly6K is associated with poor outcome in triple negative breast cancer (TNBC). Mechanistically, Ly6K is required for activation of TGF? signaling and increased expression of the immune checkpoint protein PD-L1. We propose that the biomarker Ly6K is an ideal therapeutic target for the treatment of TNBC because this protein is not expressed in normal cells, except in testis, and it is not required for vital organ function, except for spermatogenesis. Thus, targeting this protein for the treatment of TNBC, a disease affecting mostly females, is appropriate and ideal. We have identified small drug-like molecules, which specifically bind to Ly6K and inhibit in vivo tumor growth. Mechanistically, they inhibit TGF? signaling and PD- L1 expression in TNBC cells in an Ly6K dependent manner. In this proposal, we plan to validate these potential novel therapeutics in a humanized PDX model. This proposal will reveal the missing signaling links downstream of Ly6K, which activate TGF? signaling and increase PD-L1 expression. We anticipate that the findings from our research will transform the field of developmental therapeutics concerning treatment of TNBC by defining Ly6K as a novel therapeutic target for anti-TGF? signaling and inhibition of PD-L1 expression.